The present invention relates to a novel hydrogen storage alloy for use in apparatus for storing and transporting hydrogen and secondary batteries of hydrides, and also to a process for the production thereof.
Hydrogen storage alloys are materials for absorbing and desorbing a great amount of hydrogen reversibly, and binary intermetallic compounds and V-series solid solution alloys have been known, which include AB.sub.5 -type such as LaNi.sub.5, AB.sub.2 -type such as ZrMn.sub.2, AB-type such as TiFe and A.sub.2 B-type such as Ti.sub.2 Ni. Since hydrogen intrudes in the form of atom (H) into the interstices in crystal lattice of such hydrogen storage alloys to form metal hydride in an unstable bonding state, the alloys can absorb and desorb hydrogen repeatedly under a relatively mild condition to do with the temperature and the hydrogen pressure.
Further, the hydrogen absorbing characteristics of the alloys can be changed by substituting at least a portion of elements A and/or B with other elements. For example, in LaNi.sub.5 series alloys put to practical use such as hydrogen storage media or nickel-metal hydride batteries, metallographic structures and hydrogen storage characteristics are controlled so as to satisfy particular requirements in particular applications by substituting La with a misch metal (Mm) which is a mixture of rare-earth elements in the A-sites and a portion of Ni with Co, Al, Mn, etc. in the B-sites, thereby making it multiple components.
However, in the practical hydrogen storage alloys, the rechargeable hydrogen storage capacity is as small as about H/M=1 in atomic ratio between hydrogen (H) and metal (M), that is, 1 to 2% in weight ratio. Such a small rechargeable hydrogen storage capacity per unit weight gives a significant drawback as hydrogen storage media.
Mg.sub.2 Ni has been known as a lightweight hydrogen storage alloy. The alloy is superior to the hydrogen storage alloy above-mentioned in that the hydrogen storage capacity is as large as 3.6mass %. However, a high temperature is necessary to make the Mg.sub.2 Ni alloys desorb hydrogen since its hydrogen dissociation pressure is 1 atm at about 250.degree. C. This is because strongly basic Mg tends to easily donate electrons to become anionic (H.sup.-) and form a hydride in a strong bonding state. Therefore, the hydrogen dissociation pressure changes scarcely even when Mg and/or Ni are partially substituted with other elements, which is different from the LaNi.sub.5 series alloys in which hydrogen is absorbed atomically as H. Then, it has been said impossible to greatly lower the hydrogen desorbing temperature of the Mg.sub.2 Ni series alloys.
In addition to Mg.sub.2 Ni, there are several binary alloys consisting of lightweight Mg or Ca as a main ingredient and forming hydrides. However, any of the alloys above-mentioned changes into an amorphous state or decomposes disproportionately into a stable hydride such as MgH.sub.2 or CaH.sub.2. For example, as reported in the treatise (Journal of Alloys and Compounds, vol 253-254 (1997), p. 313), LaMg.sub.2 absorbs hydrogen to form LaMg.sub.2 H.sub.7, which shows a very large amount of hydrogen storage capacity in the Laves phase alloys, of H/M.gtoreq.2 in atomic ratio, that is, about 3.5% in weight ratio. However, it decomposes into La and MgH.sub.2 during the hydrogen desorption process, that is, hydrogen absorption and desorption do not progress reversibly.
Alloy hydrides are generally in a metastable state. Accordingly, the intermetallics' hydrides change into an amorphous state to be more stable thermodynamically or decompose disproportionately into hydrides of elemental metals while hydrogen absorption and desorption progress at a high temperature. LaNi.sub.5 is peculiar because it absorbs and desorbs hydrogen reversibly near room temperature, and Mg.sub.2 Ni is the only alloy that stably absorbs and desorbs hydrogen at a high temperature of 250.degree. C. Accordingly, two characteristics are required for new hydrogen storage alloys to be developed in the future: 1) No denaturation or decomposition occurs in absorbing and desorbing hydrogen and 2) hydrogen desorption is possible at room temperature, in other words, 1) high stability to hydrogenation of alloy and 2) instability of formed hydrides.
In alloys showing a high desorbing temperature, namely, low hydrogen dissociation pressure, hydrogen atoms at special interstices in crystal lattices make strongly bonds with metal atoms to form a stable hydride. Accordingly, the hydrogen desorbing temperature does not lower so much through "partial substitution method" of substituting a portion of constituent atoms with other atoms, namely, through forming a pseudo-binary alloy by merely making it having multiple components. Further, although a method of disturbing the metallographic structure by a mechanical treatment has often been also attempted in recent years, this is not effective essentially, for example, because it decreases the rechargeable hydrogen storage capacity. Accordingly, in order to enhance the stability to hydrogenation and instability of hydrides, it is necessary to strengthen bonds between metal atoms and greatly lower the chemical binding strength between metal and hydrogen atoms through drastical change of the composition and the crystal structure.